LCD technology is being developed as a possible successor to cathode ray (CRT technology for many applications. LCD technology offers important advantages, such as high reliability and reduced power, size and weight. However, in the current state of development, LCD image rendering capabilities fall short of that achievable from the use of CRT's.
There is a great need for high resolution thin film transistor/liquid crystal displays in a wide range of possible applications ranging from portable computers and test equipment to high resolution projection TV's. Such displays typically consist of a large number of picture elements (pixel) arranged in an active matrix. For a display system where the electro-optical medium is liquid crystal, each pixel is defined by a dedicated electrode on one side of two opposite transparent substrates and another electrode, which is common to all pixels and faces the viewer.
FIG. 1 illustrates in circuit form a prior art TFT liquid crystal display device. Each pixel in the active matrix is comprised of a TFT switch and a liquid crystal capacitor. The TFT gate electrodes for all the cells in a row are connected to a common horizontal gate bus, while the TFT sources in all the cells of each column are connected to a vertical data bus. The cells are addressed in a "line-at-a-time" or by line-by-line mode. By pulsing a gate bus to a positive potential relative to the source potential during the addressing interval for a particular row, the TFT's in that row are switched on. At the same time, the data signal voltages on the source busses are transferred to the TFT output electrodes (drains) and the liquid crystal capacitors. When the gate bus is switched off as the next row is addressed, the data signals are stored on the capacitors until the next addressing cycle for a particular row in the succeeding frame.
In the above described display system, the number of row and column conductors needed corresponds to the number of rows n and columns m for n.times.m pixels. In addition to the need to devote a portion of area of the display device to accommodate the row and column conductors there is also a possibility that, in view of the large number of conductors used, one or more of these conductors may be defective, rendering the display device unusable. This problem is quite common at the crossovers of row and column conductors. Obviously, the more conductors employed the greater this possibility becomes to adversely affect the yield of a large area display device.
Furthermore, the large number of row and column conductors causes problems with the production of small area display devices which are used for projection displays. Large area displays can be obtained from small area TFT liquid crystal displays by using a projection system in which the image produced by the small area display is projected onto a large area screen. However, in order to provide the desired display resolution after projection, the display device generating image should have an adequate number of row and columns of pixel density. If the number of the row and column conductors is large, a large portion of the display area is occupied by the conductors and the aperture ratio (i.e., display area where light can transmit/total area) on the display is small. Then the display exhibits low light levels.
FIG. 2 illustrates configuration of a typical prior art liquid crystal display array. Included in the array are gate bus lines 4 and source bus lines 6 which intersect in a grid-like pattern. Between the bus lines are picture elements which is usually liquid crystal held between two transparent electrodes 2. In one corner of the picture element is the switching element, a thin film transistor (TFT) 5, which either adds or removes the voltage from the liquid crystal thus making the picture element either clear or opaque. The TFT is comprised of the gate electrode 8, the source electrode 7 as well as the drain electrode 9. In this configuration, the connections for the TFT encroaches upon the normally rectangular picture element. Because of the need of hard contacts between the different bus lines, this large of a structure is necessary.
The largest reduction in aperture area for conventional AMLCD layouts is due to the need for the interconnects and contacts. Specifically, the source databus and the gate databus dramatically reduce the pixel aperture. These busses must be made larger than the minimum photolithographical dimension due to the presence of source and drain contacts at each pixel. Also, a major cause of failure of the prior art devices during their operating life is due to the contacts separating.
A further disadvantage of the prior art methods is the processing method used to construct the array. In order to provide properly conductive bus lines, metal must be used to end the array. Metal/indium tin oxide (ITO) contamination is a major source of yield loss. Also, the design of the prior art is complicated by the need for multiple layers of metal.
The present invention offers the advantages of providing a larger aperture ratio and greatly simplifying the design and fabrication of active matrix liquid crystal displays.